The invention relates to a circuit arrangement for correcting frequency errors during a transmission of data by means of data signals which are modulated according to a frequency-differential phase modulation. A regulating stage is provided which generates corrected data signals from the data signals. From the corrected data signals the transmitted data are regained by means of a modulator.
For the transmission of data via short-wave communications, it is already well-known to modulate the data signals according to a frequency-differential phase modulation. This kind of modulation is particularly suitable for short-wave communications, since two transmission channels which are situated side by side differ just slightly from one another not only with regard to the phase shifts but also to the modulation sections. In the case of the frequency-differential phase modulation, the data are, therefore, contained in the phase differences of several data signals which are simultaneously transmitted and have neighboring frequencies. The frequency band containing the frequencies of the data signals is designated as a baseband. The two frequencies limiting the baseband are the reference frequencies. In the data receiver, the data signals together with two mixed signals are converted into two group signals. A demodulator regains the transmitted data from the group signals.
In order to attain a high insensitivity with respect to frequency shifts, a circuit arrangement is necessary for correcting the frequency errors. Special demands are made on this circuit arrangement. The frequency errors relative to the center frequency of the data signals have to be compensated to less than 1 Hz. The circuit arrangement should have a large frequency range, and the regulating range and the regulating speed have to be changeable in a simple manner according to the intended application. Should the received data signals fail, the regulating condition has to be maintained. Further, radio interferences must not affect the regulation and the regulation should be carried out without using pilot signals.
It is already well-known to employ regulating loops for correcting frequency errors. With these regulating loops, in dependence upon regulating signals, the repetition frequency of an oscillator is modified. The oscillator is, for example, a voltage controlled oscillator or a quartz oscillator, at the output side of which a frequency divider having a variable dividing ratio has been connected. A circuit arrangement for correcting frequency errors in the presence of a frequency-differential phase modulation, which meets the aforesaid requirements, is hitherto not known.